It has long been known that forward biased p-n junctions in certain compound semiconductor materials will emit light. Semiconductor p-n junction diodes have been arranged in arrays to provide alphanumeric character displays. Such arrays as previously produced were relatively expensive since they involved producing the diode junction structure on a large slice of material which was thereafter cut or broken into a large number of individual diodes, thus electrically isolating them from each other, and then reassembling the diodes into arrays for arrangements to provide the characters desired. Finally, individual wire connections had to be accomplished to at least one electrode of each diode. Further, planar processing is complicated, and the resulting planar geometry is not optimum for emitting light because of internal reflection. Element nonuniformity, misalignment and bad connections cause a high percentage of rework or rejection in conventional light emitting diodes (LEDs). This fact, coupled with the labor intensive nature of the process involved as well as its inherently high rated material waste, made LED displays relatively expensive to produce, and difficult to fabricate large displays having good resolution (a high number of lighted points per unit area).
Other displays as liquid crystal and electrochromic require ambient light, and are likely to have lifetime problems and be temperature sensitive. Electroluminescent displays require high voltage as do plasma displays.
To overcome the weaknesses of the conventional LED display a light emitting diode taking the form of a sphere or spheroid has been used. In U.S. application Ser. No. 752,389 entitled "Light Emitting Diode Element in Character Display" by G. F. Wakefield a spherical light emitting element having a multilayer sphere forming a subsurface p-n junction inserted into an insulation member is disclosed.
In pending application TI-6782, Ser. No. 834,630, filed Sept. 19, 1977, entitled "Optoelectronic Displays Using Uniformly Spaced Arrays of Semi-Sphere Light-Emitting Diodes and Method of Fabricating Same" by the inventor of the present invention and assigned to the assignee of the present invention, diode elements in the form of semi-spheres having an outer layer of semiconductor material of one conductivity type and a subsurface layer of semiconductor material of an opposite conductivity type are mounted to a surface of insulating substrate member extended therefrom to allow a portion of each diode element to protrude past the outer surface of the substrate.